Perp could have had a change of clothes. Did the murders, waded across the narrow creek, changed clothes, put the soiled clothes in the backpack, then slipped out of the park, out of sight of the bridge. Edit: Or changed clothes after the murders, and slipped out via the cemetery. I keep getting the orientation of the bridge, the screen grabs, and the crime scene jumbled up.
People have brought up his gait on the bridge. I've maintained he was exaggerating his movements a little. Why? Maybe it's not a disability at all, he has to make more effort to stay upright/erect because of the heaviness of the items on his torso. Even a little weight there would require more effort to stay upright, especially on an old bridge.
Conceal the backpack, that way nobody would notice it post-crime, had they seen him earlier. Shove the soiled clothes in it, after removing the clean ones. Could those be crocs on his feet? Easier to wade with them.
Does what I wrote seem plausible?